The communications industry is rapidly changing to adjust to emerging technologies and ever increasing customer demand. This customer demand for new applications and increased performance of existing applications is driving communications network and system providers to employ networks and systems having greater speed and capacity (e.g., greater bandwidth). In trying to achieve these goals, a common approach taken by many communications providers is to use packet switching technology. Increasingly, public and private communications networks are being built and expanded using various packet technologies, such as Internet Protocol (IP). Note, nothing described or referenced in this document is admitted as prior art to this application unless explicitly so stated.
Measurement and control systems are widely used in traditional test and measurement, industrial automation, communication systems, electrical power systems and many other areas of modern technology. The timing requirements placed on these measurement and control systems are becoming increasingly stringent. Traditionally these measurement and control systems have been implemented in a centralized architecture in which the timing constraints are met by careful attention to programming combined with communication technologies with deterministic latency. In recent years an increasing number of such systems utilize a more distributed architecture and increasingly networking technologies having less stringent timing specifications than the older more specialized technologies. In particular, Ethernet communications are becoming more common in measurement and control applications. This has led to alternate means for enforcing the timing requirements in such systems. One such technique is the use of system components that contain real-time clocks, all of which are synchronized to each other within the system. This is very common in the general computing industry. For example, essentially all general purpose computers contain a clock. These clocks are used to manage distributed file systems, backup and recovery systems and many other similar activities. These computers typically interact via LANs and the Internet. In this environment the most widely used technique for synchronizing the clocks is the Network Time Protocol, NTP, or the related SNTP.
It is not uncommon for the timing accuracies to be required to be in the sub-microsecond range, with the requirement that this synchronization must be available on a range of networking technologies including Ethernet as well as other technologies found in industrial automation and similar industries. IEEE 1588 addresses the clock synchronization requirements of measurement and control systems. However, especially when used in conjunction with Ethernet and other local area network technologies, there can be jitter in the synchronization, which leads to a larger inaccuracies among clocks.
FIG. 1 illustrates a prior art configuration for associating timestamps with frames. Shown is a hardware-assist circuit which monitors the 125 MHZ GMII transmit and receive signals between the MAC and PHY. Two independent PTP message detection logic blocks facilitate time stamping of transmit and receive paths. The hardware assist uses a frequency compensated clock to keep system time. In this circuit a 32-bit accumulator adds the contents of addend register to itself at the frequency of 100 MHz. The 64-bit system clock counter is incremented whenever the accumulator overflows. This provides a high precision frequency compensated clock that can be tuned to one part per billion compensation accuracy.